2.2 The Zeroth Law#
The zeroth law states that if two thermodynamic systems are in thermal equilibrium with each other, and also separately in thermal equilibrium with a third system, then the three systems are in thermal equilibrium with each other.
Fig. 28 Development of a thermal equilibrium in a closed system over time through heat flow removing temperature differences. Image source: Thermal equilibrium - Wikipedia#
Two systems are said to be in thermal equilibrium if they are linked by a wall permeable only to heat, and they do not change over time (Fig. 28). This property is applied in a basic thermometer. When a thermometer is placed in a room or on a body, the temperature reading on the thermometer changes until thermal equilibrium is established between the thermometer and the surrounding system.
Fig. 29 Mercury thermometer (mercury-in-glass thermometer) for measurement of room temperature. Image source: Thermometer - Wikipedia#
Systems in thermodynamic equilibrium are always in thermal equilibrium, but the converse is not always true. If the connection between the systems allows transfer of energy as ‘change in internal energy’ but does not allow transfer of matter or transfer of energy as work, the two systems may reach thermal equilibrium without reaching thermodynamic equilibrium. In thermodynamic equilibrium, there are no net macroscopic flows of matter or energy within a system or between systems. In a system that is in its own state of internal thermodynamic equilibrium, no macroscopic change occurs.